This invention relates generally to ground isolation devices and particularly to marine ground isolators suitable for use in the ground connection between the ground conductor of a land power system and conductive portions of marine structures.
A boat setting in the water and having at least one conductive surface beneath the water line has the capability of supplying or taking current from the water. This capability is dependent upon the conductivity of the water and the amount of exposed conductor on the boat. Since nearly any boat will have at least a propeller and shaft in the water, there can generally be some conduction between the metal of the boat and the water. The boat will act as a cathode or an anode depending on the potential of the conductive surface relative to the water. In general, if the conductive surface is positive to the water, it can be considered an anode; and if current is allowed to flow, parts of the conductive metal will go into solution in the water and the metal will therefore corrode. The higher the potential and the more current that flows, the larger the amount of corrosion. The corrosion can be reduced by maintaining the potential of the conductive surface of the boat negative with respect to the water. Specifically, if the potential is maintained at 0.8 volts or more negative in respect to the water, all corrosion for most metals will be stopped. This is the well-known basis for cathodic protection against galvanic corrosion.
A problem exists whenever the exposed parts of the boat are connected through suitable current-carrying conductors to any other item which has a potential reference to the water. An example would be when the boat is electrically tied to the earth ground, a ground line for an electrical system, or a conductive dock. In this case, the boat may be driven positive or negative with respect to the water, depending on potential of the external object to which it is connected. For example, if the exposed conductive portions of a boat are connected to a shore ground line, which itself is 1 volt positive with respect to the water, the boat will then try to also be 1 volt positive with respect to the water and current will flow with the conductive surfaces acting as an anode, and therefore corroding.
There are two solutions to this problem: First, an equal and opposite potential could be introduced between the conductive surfaces of the boat and the ground power connection, but this potential source must be capable of supplying sufficient current to override the external source which was causing the original difference in voltage. Second, the connection from the boat to the shore ground connection can be broken. At the present time, the second solution is the one which is generally used by boats in marinas around the world.
Many boats also are connected to sources of 110 or 220 volt ac shore power while they are at the dock. Unless sophisticated and expensive isolation transformers are used, a safety problem can exist if the ground connection from the boat to the shore ground is removed. Any leakage from any electrical item on the boat from the shore power connection to the boat's conductive surface can raise these surfaces to a dangerous potential with respect to the surrounding water and dock. It is estimated that any voltage over 21/2 volts rms could conceivably be dangerous when contacted by a person emersed in salt water. If no ground connection is used, it is also possible that conductive portions of the boat above the water line could become potentially dangerous with respect to a metal dock or the water itself. For the above reasons, it is desired to be able to maintain a shore ground connection to all exposed conductive portions of the boat whenever ac shore power is connected.
One present type of marine ground isolator uses silicon rectifiers arranged so that the intrinsic junction voltage drop of the diodes prevents current flow due to potentials below a preselected value; eg see U.S. Pat. No. 3,769,926. This diode junction voltage offset technique is also implemented in U.S. Pat. No. 3,636,409 which further incorporates capacitors shunting the diodes so as to provide an additional ground path for ac currents.
The above described isolators have proven quite satisfactory in many applications. However, a significant aspect of the subject invention is the implementation of a new and improved approach whereby ac and dc potentials across the "isolated" ground connection are separately monitored and used to control a switching arrangement in series with the ground connection.